Vehicles such as automobiles employ a sub-frame structure that is fixed to front side frames as vehicle body members and to which suspension components, such as suspension arms and a stabilizer, are attached and that supports the suspension components.
As this type of sub-frame structure, for example, Japanese Unexamined Patent Application Publication No. 2007-302147 discloses a sub-frame structure including a rear member that is made of light metal and to which suspension components are attached, two side members that are made of steel and are joined to the front ends of two side portions of the rear member and extend toward the front of the vehicle, and a cross member that connects the two side members in the vehicle width direction.
Japanese Unexamined Patent Application Publication No. 2009-126472 discloses, in relation to a welded structure of an automotive structure such as a center pillar, friction stir welding a flange on the opening-side edge of a box structure member made of aluminum alloy and a flat plate-like cover made of galvanized steel plate.